Automatic exposure (AE) is a process by which a still camera or a video camera controls an overall lightness of a picture. For conventional digital cameras, an aperture, a shutter time and a gain are controlled to adjust the lightness. A gain of six decibels (dB) increases a signal voltage by a factor of two.
Many combinations of aperture, shutter time, and gain can achieve a given lightness level. Therefore, besides determining the overall lightness level, conventional AE methods operate with the following considerations: (i) all lenses have a maximum aperture and a minimum aperture, (ii) cameras have a minimum shutter time and a maximum shutter time, (iii) for video cameras, the shutter time cannot exceed a frame time, (iv) electro-optical sensors have a minimum gain and a maximum gain, (v) a wide aperture reduces a depth-of-field, which can be helpful (i.e., focus on the subject and blur the background) or not helpful (i.e., less of the scene can be in focus), (vi) a lens will typically achieve a maximum sharpness and a lowest distortion over a range of aperture values that is smaller than the full range of possible aperture values, (vii) a long shutter time increases motion blurriness, which can be good (i.e., show motion in a still picture, make motion appear smooth in a video sequence) or bad (i.e., the picture is too blurry), (viii) high gain increases noise and is therefore usually avoided, (ix) for low light situations, and with the above described limitations of using shutter time and aperture to increase light, a high gain is sometimes appropriate.
With reasonably good light, conventional AE techniques will try to keep the total picture lightness at a certain level, independent of the amount of available light. For low illumination levels, the conventional AE techniques will permit the picture lightness to be reduced. Furthermore, the maximum gain used at the low illumination levels is reduced to keep the pictures from being too noisy.